Coating on dielectric insert of a resonant RF cavity

ABSTRACT

Disclosed herein are radio frequency (RF) cavities and systems including such RF cavities. The RF cavities are characterized as having an insert with at least one sidewall coated with a material to prevent charge build up without affecting RF input power and that is heat and vacuum compatible. One example RF cavity includes a dielectric insert, the dielectric insert having an opening extending from one side of the dielectric insert to another to form a via, and a coating layer disposed on an inner surface of the dielectric insert, the inner surface facing the via, wherein the coating layer has a thickness and a resistivity, the thickness less than a thickness threshold, and the resistivity greater than a resistivity threshold, wherein the thickness and resistivity thresholds are based partly on operating parameters of the RF cavity.

The present application claims the priority benefit of U.S. ProvisionalPatent Application Ser. No. 62/884,874, filed Aug. 9, 2019. Thedisclosures of the foregoing application are incorporated herein byreference.

FIELD OF THE INVENTION

This application is generally directed toward radio frequency cavities,and more specifically to coating layers in radio frequency cavities.

BACKGROUND OF THE INVENTION

Charged particle microscopes typically provide a continuous chargedparticle beam for an energized emitter. While a continuous beam worksfine in most applications, a time-varying beam would be beneficial inother applications, such as time-dependent measurements where the sampleis being excited while being imaged. Time-varying charged particlebeams, however, are more difficult to generate. One known method is touse an optically-pumped emitter that emits charged particles only whenoptical energy is radiating the source. While this technique istypically only used in a pulsed mode, continuous charged particle beamsare possible but come with unwanted drawbacks in operation. Although adedicated pulsed system is useful in some applications, it is desirableto have a charged particle microscope that can provide both pulsed andcontinuous charged particle beams.

SUMMARY

Disclosed herein are radio frequency (RF) cavities and systems includingsuch RF cavities. The RF cavities are characterized as having an insertwith at least one sidewall coated with a material to prevent chargebuild up without affecting RF input power and that is heat and vacuumcompatible. One example RF cavity includes a dielectric insert, thedielectric insert having an opening extending from one side of thedielectric insert to another to form a via, and a coating layer disposedon an inner surface of the dielectric insert, the inner surface facingthe via, wherein the coating layer has a thickness and a resistivity,the thickness less than a thickness threshold, and the resistivitygreater than a resistivity threshold, wherein the thickness andresistivity thresholds are based partly on operating parameters of theRF cavity

Another example apparatus includes an outer shell, the outer shellhaving an internal cavity with openings on a top and bottom of theinternal cavity, an insert disposed within the internal cavity of theouter shell, the insert having a via extending all the way through andaligned with the openings on the top and bottom of the outer shell, anda coating layer disposed on a surface of the insert facing the via, thecoating layer having a thickness less than a thickness threshold and aconductance less than a conductance threshold, wherein the coating layerprevents charge buildup on the insert.

An example system at least includes an emitter coupled to emit a beam ofelectrons along an optical path, one or more sets of optics arrangedalong the optical path; and an RF cavity encompassing a portion of theoptical path, the RF cavity comprising, a dielectric insert, thedielectric insert having an opening extending from one side of thedielectric insert to another to form a via, and a coating layer disposedon an inner surface of the dielectric insert, the inner surface facingthe via, wherein the coating layer has a thickness and a resistivity,the thickness less than a thickness threshold, and the resistivitygreater than a resistivity threshold, wherein the thickness andresistivity thresholds are based partly on operating parameters of theRF cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example charged particle microscope in accordance with anembodiment of the present disclosure.

FIGS. 2A, 2B and 2C are example illustrations of an RF cavity inaccordance with an embodiment of the present disclosure.

FIG. 3 is an example illustration of a sidewall of an RF cavity inaccordance with an embodiment of the present disclosure.

Like reference numerals refer to corresponding parts throughout theseveral views of the drawings.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention are described below in the contextof a charged particle microscope where a radio frequency cavity isenabled to strobe a charged particle beam. For example, a radiofrequency cavity is included in a transmission electron microscope sothat an electron beam may be strobed so that a sample is illuminatedwith a train of electron beam pulses instead of a continuous electronbeam. However, it should be understood that the methods described hereinare generally applicable to a wide range of different charged particlemicroscopes.

As used in this application and in the claims, the singular forms “a,”“an,” and “the” include the plural forms unless the context clearlydictates otherwise. Additionally, the term “includes” means “comprises.”Further, the term “coupled” does not exclude the presence ofintermediate elements between the coupled items.

The systems, apparatus, and methods described herein should not beconstrued as limiting in any way. Instead, the present disclosure isdirected toward all novel and non-obvious features and aspects of thevarious disclosed embodiments, alone and in various combinations andsub-combinations with one another. The disclosed systems, methods, andapparatuses are not limited to any specific aspect or feature orcombinations thereof, nor do the disclosed systems, methods, andapparatus require that any one or more specific advantages be present orproblems be solved. Any theories of operation are to facilitateexplanation, but the disclosed systems, methods, and apparatus are notlimited to such theories of operation.

Although the operations of some of the disclosed methods are describedin a particular, sequential order for convenient presentation, it shouldbe understood that this manner of description encompasses rearrangement,unless a particular ordering is required by specific language set forthbelow. For example, operations described sequentially may in some casesbe rearranged or performed concurrently. Moreover, for the sake ofsimplicity, the attached figures may not show the various ways in whichthe disclosed systems, methods, and apparatus can be used in conjunctionwith other systems, methods, and apparatus. Additionally, thedescription sometimes uses terms like “produce” and “provide” todescribe the disclosed methods. These terms are high-level abstractionsof the actual operations that are performed. The actual operations thatcorrespond to these terms will vary depending on the particularimplementation and are readily discernible by one of ordinary skill inthe art.

In some examples, values, procedures, or apparatuses are referred to as“lowest”, “best”, “minimum,” or the like. It will be appreciated thatsuch descriptions are intended to indicate that a selection among manyused functional alternatives can be made, and such selections need notbe better, smaller, or otherwise preferable to other selections.

One solution to the problem described above is to include a radiofrequency (RF) cavity within a charged particle microscope. The RFcavity may be used to generate a pulsed charged particle beam throughinteraction of one or more RF standing waves and a passing chargedparticle beam. In some embodiments, the RF cavity may include an inputand output aperture that allows the charged particle beam to internallytraverse the RF cavity so that the charge particle beam interacts withthe RF waves established within the cavity. In other embodiments, a“chopping aperture”, which is where the pulses are created due tointeraction of the RF standing waves and the charged particle beam atthe aperture, is arranged away from the actual RF cavity. In eitherembodiment, however, the space and time interaction of the aperture, theRF standing wave(s) and the charged particle beam form the chargedparticle beam pulses. In such a technique, the RF waves will effectivelymove the charged particle beam across the output aperture so that apulsed train of charged particles are generated, which are then directedtoward a sample.

Previously, a dual-mode RF cavity as a beam chopper in a chargedparticle microscope has been described, such as in U.S. Pat. No.9,048,060, which is incorporated herein for all purposes. As discussedin that reference, the RF cavity could include a dielectric insert,where the insert could be coated by a film of electrically conductivematerial for prevention or reduction of charge accumulation on thedielectric insert. While such an observation appears on its surface asstraightforward, the implementation of such an idea required more effortthan anticipated.

The additional effort stemmed from the realization that the coatingmaterial would need to meet very specific requirements. In general, thecoating material needs to be sufficiently conductive to prevent thecharge accumulation, but should not affect the resonance of the RFcavity nor should it generate substantial power loss. To prevent effectson the resonance of the RF cavity, the thickness of the coating layershould be thin enough that the RF power transmits through withoutcausing much reflection and/or absorption and thin enough that theresonance of the RF cavity is not adversely affected. To meet such arequirement, it is desirable that the thickness of the coating layer beless than a skin depth, where the operating parameters of the RF cavityalong with the resistance of the material determine the skin depth.While the thickness is desirably much thinner than the skin depth, theconductance/resistivity of the material will also affect the allowablethickness.

With regards to the power dissipation requirement, it is desirable thatthe coating material generate negligible power dissipation with respectto input RF power. For example, a maximum power dissipation equal to 10%of the input power is desired, but less power dissipation would bepreferable. The power dissipation is determined by the coating materialcharacteristics, thickness and operating parameters of the RF cavity aswell. If the requirements are satisfied, then the quality factor of theRF cavity should be maintained.

While deciding on a material seems trivial, that turned out to not bethe case. For example, conductive and semi-conductive materials willstop the accumulation of charge, but their high conductivities result inextremely thin layers that are not manufacturable to the degree requiredfor rough surfaces. For example, a copper layer would need to be muchless than 1.2 μm (the skin depth at a 3 GHz frequency) to satisfy thethickness requirement. However, even if a 10 nm copper layer wasmanufacturable, the associated power dissipation, e.g., 50 kW, does notsatisfy the power dissipation requirement. In such an example, the powerdissipation of the copper layer may be all or most of the input power,which is highly undesirable. Semiconductor materials do not satisfy therequirements either even though they may allow for thicker layers at alower power dissipation than copper.

In general, it has been determined that the coating layer needs to havea high resistance/low conductance that allows for a layer of amanufacturable thickness with minimal power loss at the RF cavityoperating parameters. Additionally, it is desirable that the materialwithstand high temperature environments (up to 1000° C., for example),be homogenous/continuous in manufacturing, and compatible with highvacuum environments, e.g., no outgassing. One solution is amorphoussilicon (a-Si), which has a resistance greater than 60 GΩ, allows for athickness of up to 500 nm (and possibly thicker) while providingnegligible power dissipation, microwatts of power dissipation forexample. While other materials may also meet the requirements, such assilicon carbide and zinc oxide for example, the disclosure will focus ona-Si for illustration purposes.

FIG. 1 is an example charged particle microscope 100 in accordance withan embodiment of the present disclosure. The charged particle microscope(CPM) 100 includes a radio frequency (RF) cavity that a charged particlebeam passes through for the intended purpose of dynamically strobing thecharged particle beam to generate a stream of charged particle beampulses for illuminating a sample. While strobing the charged particlebeam is disclosed, the RF cavity may be disabled when desired so that acontinuous charged particle beam is provided to a sample instead. Insome embodiments, the CPM 100 is a transmission electron microscope(TEM), but the type of CPM is not limiting. In other embodiments, CPM100 may be a scanning electron microscope (SEM), scanning transmissionelectron microscope (STEM), a dual beam microscope that includes both anSEM and a focused ion beam (FIB), or the like.

The simplified CPM 100 illustrated in FIG. 1 includes an emitter 102,one or more collimator lenses 104A and 104B, a beam defining aperture(BDA) 106, an RF cavity 108, an objective lens 110A and 110B, a sampleholder 112, one or more projection lenses 114A and 114B, and a detector116. One skilled in the art understands that additional or fewercomponents may be implemented in CPM 100 and the illustrated componentsare mere examples of what may typically be implemented in a TEM.

The emitter 102 provides a beam of charged particles 118 that isdirected down the CPM 100 on an optical path that traverses the othercomponents, ending on the detector 116. The collimator lenses 104A, Bcondition and focus the charged particle beam 118 onto a desiredlocation/plane, such as the plane where the sample holder 112 islocated. The objective lens 110A, B additionally focuses the chargedparticle beam 118 onto the sample holder location and further conditionthe charged particle beam 118 as it emerges from a sample. After thecharged particle beam 118 emerges from a sample, e.g., after traversinga sample, the projection lenses 114A, B condition the emergent chargedparticle beam 118 for detection by detector 116. The detector 116detects images of the sample based on the interaction of the chargedparticles of the charged particle beam 118 with the sample.

In general, the various lenses 104, 110 and 114 may be electrostatic,electromagnetic, or combinations thereof. Additionally, the emitter 102may be any type of emitter, such as thermionic, cold field emission, andthe like. In most embodiments, emitter 102 provides electrons, but ionemitters are also possible and within the bounds of the presentdisclosure. The detector 116 may be a direct electron detector or amicro-channel plate based detector as known in the art.

The RF cavity 108 includes a via extending all the way through thecavity so that the charged particle beam 118 can pass through andcontinue traversing the CPM 100 along the optical path. The RF cavity108 may be any shape desired, such as round, oval, square, rectangular,and the like, that enables one or more standing waves, e.g., modes, tobe established. In general, it is desirable that the dimensions of theRF cavity 108 be based on one or more desired operating frequencies sothat a resonant cavity is established at the one or more desiredoperating frequencies. Depending on the number of modes desired to beestablished within the RF cavity 108, an associated number ofemitter-antenna pairs will be included. The RF cavity 108 will includean outer chamber designed to establish the desired modes, and adielectric insert. The dielectric insert (discussed in further detailbelow) will also include a via extending through it for passage of thecharged particle beam 118. As such, the dielectric insert will bearranged inside the RF cavity 108 to encompass the charged particle beamoptical axis.

The RF cavity 108, as noted, may be enabled to strobe the chargedparticle beam 118 so that a train of charged particle beam pulses emitfrom the RF cavity, where the pulse separation is determined by theoperating frequency of the RF cavity 108, and the pulse width/durationis determined by a combination of the input power, quality factor andfrequency of the RF cavity 108. Such an RF cavity is further describedin U.S. Pat. No. 9,048,060 B2, which is owned by the current applicant,and which is incorporated herein for all purposes. Other than simplyforming a strobed charged particle beam, the RF cavity 108 may be usedin systems that include a streak camera that allows for time-resolvedcharged particle microscopy. In general, it is desirable that the RFcavity 108 does not build up charge on the dielectric insert due to thepassing charged particle beam 118. The buildup of charge causes thepassing charged particle beam 118 to drift out of alignment, which mayreduce or stop the charged particle beam 118 from continuing along theoptical path or exiting from the RF cavity 108.

To reduce or eliminate the buildup of charge on the dielectric insert,it is desirable to coat the insert, at least on sidewalls facing thevia, with a conductive yet highly resistive material. It is desirablethat this material be of a thickness that will not affect the resonanceof the cavity, and further not dissipate substantial RF power. Forexample, the thickness may be less than a skin depth at the RF operatingparameters and the power dissipation be minimal, e.g., 10% or less of RFinput power. Further, it may also be desirable that the coating materialbe compatible with high vacuum environments (e.g., little to nooutgassing) and resistant to high temperatures. In some embodiments, thedesired thickness may be much less than the calculated skin depth, whichmay be a manufacturing concern if a continuous layer is desired. Givensuch requirements, a material that is vacuum compatible with a highresistance/low conductance is desirable. Conductive materials andsemiconductors due not provide the desired results due to the determinedthickness and power dissipation being either un-manufacturable and/ortoo high, respectively.

One solution, however, is amorphous silicon (a-Si). Amorphous siliconhas a high resistance/low conductance, which allows for thicker, moremanufacturable layers that have negligible power dissipation. Note, theskin depth has a square root relationship with resistivity (ρ) and thepower dissipation has a linear relationship with conductance (σ), whereσ=1/ρ. Additionally, a-Si meets the other requirements, such as hightemperature compatible, high vacuum compatible, and thin, continuous,and homogenous layers are easily manufacturable using chemical vapordeposition, for example. As such, a coating layer formed from a-Siprevents the buildup of charge on the sidewalls of the insert and has aresistance/conductance characteristic that allows for relatively thicklayers that generate minimal power dissipation.

FIGS. 2A through 2C are example illustrations of RF cavity 206 inaccordance with an embodiment of the present disclosure. The RF cavity206 may be implemented in a CPM system, such as CPM 100, for deflectinga charged particle beam using one or more RF standing waves generatedwithin the RF cavity 206. Deflecting the charged particle beam resultsin a pulsed charged particle beam as an output of the aperture thatextends through the RF cavity 206. A pulsed charged particle beam may,for example, allow for time-resolved probing of a sample.

The RF cavity 206 includes an outer shell 220, an insert 222, one ormore conductors 224A, 224B, and a coating layer 228. The outer shell 220may be any desired shape, such as round, oval, square, and the like, andforms a cavity 234 of a similar geometry. Additionally, the outer shell220 includes an opening 230 in a top surface 232 (and a bottom surface,not shown) that extends through the entire height and the cavity 234 ofthe outer shell 220. In the embodiment shown in FIG. 2A, the outer shell220 has a height h and an inner diameter d1, where the opening/via has adiameter r. In some embodiments, the RF cavity 206 has a resonance thatis determined, at least partially, by the inner diameter d2.

The insert 222 may be located at a center of the outer shell 220 and mayhave a height to fit within the cavity 234, and has a diameter d2 thatis less than d1 of the outer shell 220. Additionally, the insert 222also has an opening/via that extends through the height of the insert222. In general, the via in the insert 222 aligns with the top andbottom openings in the outer shell 220 so that charged particle beam canpass through the RF cavity 206. The insert 222 may be formed from adielectric, such as ZrTi04, for example, but other dielectric materialsare also implementable.

The one or more conductors 224 A, B, which may alternatively be referredto as antennae, may allow the formation of RF standing waves inside theRF cavity 206. the two or more conductors 224 A, B may be formed fromany conductor, such as copper, silver, gold, platinum, etc., and arecoupled to one or more RF power sources (not shown) for the generationof a desired standing wave. While the RF cavity 206 only shows one pairof antennae, in other embodiments, four conductors may be included forthe formation of two standing waves. In such an embodiment, the secondpair of antennae may be included so that the additional standing wave isat a right angle to the first standing wave.

The coating layer 228 may be at least formed on an inner surface 226 ofthe insert 222. In general, the coating layer 228 is included to reduceor prevent charge buildup on the insert 222 due to the passing chargedparticle beam. However, as noted above, it is desirable that theinclusion of the coating layer 228 not affect the resonance of the RFcavity 206 and that any RF power dissipation due to the coating layer228 be minimal. To prevent changes of the resonance, the coating layer228 may desirably be less than a skin depth. The skin depth determinedbased on the RF cavity operating parameters, such as 3 GHz operatingfrequency for example. With regards to the power dissipation, the powerdissipation of the coating layer 228 may desirably be 10% or less of theinput power. While various characteristics of the coating layer 228material affect skin depth and power dissipation, at least onecharacteristic can affect both—conductance/resistivity of the selectedmaterial.

In terms of conductance/resistivity, the skin depth is dependent uponthe square root of resistivity, whereas the power dissipation has alinear relationship with conductance. If conductance is the simpleinverse of resistivity, then as the conductance decreases and theresistivity increases, the skin depth increases and the powerdissipation decreases. This relationship is then used to determine amaterial to use for the coating layer 228. In some embodiments, thethickness should be less than a thickness threshold and the resistivityshould be greater than a resistivity threshold (conductance less than aconductance threshold). These thresholds, as long as they are satisfied,ensure the requirements of the coating layer 228 are satisfied, i.e.,the thickness and power dissipation requirements.

As noted, the two main characteristics desired of the coating layer 228is a manufacturable skin depth and negligible/minimal power dissipation.Manufacturable implies that a continuous, homogenous and reproduciblelayer may be deposited on a surface that has a root-mean-square (rms)roughness on the order of 1 to 2 microns. Additionally, the desiredthickness of the coating layer 228 is much less than the skin depth atthe RF operating parameters, where much less refers to 10% or less ofthe calculated skin depth. The power dissipation is desired to benegligible with regards to input RF power, and refers to 10% or less ofthe input power. In terms of thresholds, the thickness threshold is atmost the skin depth, but could be less than a skin depth, and theresistivity/conductance thresholds may be based on either a multiplierof conductance and the thickness threshold or determined based on 10% ofthe input RF power. For example, If you take 10 nm as the smallestviable coating thickness for manufacturing reasons, then the materialfor the coating layer 228 would need to have a resistivity ρ>5×10⁻²Ωcm(but preferably much higher). In general terms, theconductance/resistivity thresholds may be determined based on backcalculating a thickness for a given geometry and at 10% of the inputpower, while the thickness threshold is based on the skin depth.

Additionally, the material for the coating layer 228 is desired to meetother requirements, such as resistance to high temperatures and bevacuum compatible. To satisfy the temperature resistance requirement,the material should not change, melt, or sublimate at high temperatures,600 to 1000° C. for example. And to satisfy the vacuum compatiblerequirement, the material should not outgas under high vacuum.

One material that meets all of the above requirements is amorphoussilicon. Amorphous silicon has been found to be highly resistive,greater than 60 GΩ (ρ>5×10³ Ω·m). At this resistance/conductivity, theskin depth is 0.65 μm, which allows for a layer thickness of up to 500nm. A layer of 500 nm generates power dissipation of 9 μW, which isclearly negligible for an input power of 1 W or more. Amorphous siliconeasily meets the temperature and vacuum compatibility requirements aswell due to its high temperature and high vacuum compatibility.

FIG. 3 is an example illustration of a sidewall 326 in accordance withan embodiment of the present disclosure. The illustrated sidewall 326 isone example of the sidewall 226 of RF cavity 206. The sidewall 226includes insert 322 and coating layer 328. The coating layer 328 mayhave a thickness dependent upon a skin depth of the material based onworking parameters of an RF cavity. For example, the thickness may be100 to 500 nm, but should be thick enough to provide a continuous layeron the sidewall 326 of the insert 322.

The embodiments discussed herein to illustrate the disclosed techniquesshould not be considered limiting and only provide examples ofimplementation. Those skilled in the art will understand the othermyriad ways of how the disclosed techniques may be implemented, whichare contemplated herein and are within the bounds of the disclosure.

Disclosed herein is a radio frequency (RF) cavity comprising adielectric insert, the dielectric insert having an opening extendingfrom one side of the dielectric insert to another to form a via, and acoating layer disposed on an inner surface of the dielectric insert, theinner surface facing the via, wherein the coating layer has a thicknessand a resistivity, the thickness less than a thickness threshold, andthe resistivity greater than a resistivity threshold, wherein thethickness and resistivity thresholds are based partly on operatingparameters of the RF cavity.

The example RF cavity of above where the thickness threshold is lessthan a skin depth of the coating layer, the skin depth based on aresistivity of the material of the coating layer and the operatingparameters.

The example RF cavity of above where the thickness does not affect aresonance of the RF cavity.

The example RF cavity of of above where the resistivity threshold isbased on a percentage of input power.

The example RF cavity of above where a power dissipation based on theresistivity is equal to or less than ten percent of the input powerprovided to the RF cavity.

The example RF cavity of above where the material is amorphous silicon.

The example RF cavity of above further including an outer shell, theouter shell including a via extending all the way through and alignedwith the via of the insert.

Also disclosed herein is an example apparatus comprising an outer shell,the outer shell having an internal cavity with openings on a top andbottom of the internal cavity, an insert disposed within the internalcavity of the outer shell, the insert having a via extending all the waythrough and aligned with the openings on the top and bottom of the outershell, and a coating layer disposed on a surface of the insert facingthe via, the coating layer having a thickness less than a thicknessthreshold and a conductance less than a conductance threshold, whereinthe coating layer prevents charge buildup on the insert.

The example apparatus of above where the thickness threshold is lessthan a skin depth of the coating layer, the skin depth based on one ormore material properties of a material forming the coating layer, and onoperating parameters of the apparatus.

The example apparatus of above where the thickness threshold isdetermined so that a radio frequency (RF) resonance of the outer shellis not impacted.

The example apparatus of above where the conductance threshold is basedon a percentage of input power and the thickness of the coating layer.

The example apparatus of above where a power dissipation of the coatinglayer is less than ten percent of input power.

The example apparatus of above where the coating layer is amorphoussilicon.

The example apparatus of above where the coating layer is 100 to 500 nmthick and has a conductance less than 1.6×10⁻³Ω⁻¹ m⁻¹.

Also disclosed herein is an example system comprising an emitter coupledto emit a beam of electrons along an optical path, one or more sets ofoptics arranged along the optical path, and an RF cavity encompassing aportion of the optical path. The RF cavity comprising a dielectricinsert, the dielectric insert having an opening extending from one sideof the dielectric insert to another to form a via; and a coating layerdisposed on an inner surface of the dielectric insert, the inner surfacefacing the via, wherein the coating layer has a thickness and aresistivity, the thickness less than a thickness threshold, and theresistivity greater than a resistivity threshold, wherein the thicknessand resistivity thresholds are based partly on operating parameters ofthe RF cavity.

The example system of above where the thickness threshold is less than askin depth of the coating layer, the skin depth based on a resistivityof the material of the coating layer and the operating parameters.

The example system of above where the resistivity threshold is based ona percentage of input power.

The example system of above where a power dissipation based on theresistivity is equal to or less than ten percent of the input powerprovided to the RF cavity.

The example system of above where the material is amorphous silicon.

The example system of above further including an outer shell, the outershell including a via extending all the way through and aligned with thevia of the insert.

What is claimed is:
 1. A radio frequency (RF) cavity comprising: adielectric insert, the dielectric insert having an opening extendingfrom one side of the dielectric insert to another to form a via; and acoating layer disposed on an inner surface of the dielectric insert, theinner surface facing the via, wherein the coating layer is amorphoussilicon.
 2. The RF cavity of claim 1, wherein a thickness of the coatinglayer is less than a skin depth at an RF operating frequency of 3 GHz.3. The RF cavity of claim 2, wherein the thickness of the coating layeris less than 0.65 microns.
 4. The RF cavity of claim 1, wherein thethickness of the coating layer is 500 nanometers.
 5. The RF cavity ofclaim 1, further including an outer shell, the outer shell including avia extending all the way through and aligned with the via of theinsert.
 6. An apparatus comprising: an outer shell, the outer shellhaving an internal cavity with openings on a top and bottom of theinternal cavity; an insert disposed within the internal cavity of theouter shell, the insert having a via extending all the way through andaligned with the openings on the top and bottom of the outer shell; anda coating layer disposed on a surface of the insert facing the via, thecoating layer being formed of amorphous silicon, wherein the coatinglayer prevents charge buildup on the insert.
 7. The apparatus of claim6, wherein the thickness threshold is less than a skin depth of thecoating layer, the skin depth based on one or more material propertiesof the coating layer, and on operating parameters of the apparatus being3 GHz.
 8. The apparatus of claim 7, wherein the thickness of the coatinglayer is less than 0.65 microns.
 9. The apparatus of claim 6, whereinthe a thickness of the coating later is 500 nanometers.
 10. Theapparatus of claim 6, wherein the coating layer is 100 to 500 nm thickand has a conductance less than 1.6×10⁻³Ω⁻¹ m⁻¹.
 11. A systemcomprising: an emitter coupled to emit a beam of electrons along anoptical path; one or more sets of optics arranged along the opticalpath; and an RF cavity encompassing a portion of the optical path, theRF cavity comprising; a dielectric insert, the dielectric insert havingan opening extending from one side of the dielectric insert to anotherto form a via; and a coating layer disposed on an inner surface of thedielectric insert, the inner surface facing the via, wherein the coatinglayer is formed of amorphous silicon.
 12. The system of claim 11,wherein the thickness of the coating layer is less than a skin depth ofthe coating layer, the skin depth based on a resistivity of the materialof the coating layer and the operating parameters.
 13. The system ofclaim 11, further including an outer shell, the outer shell including avia extending all the way through and aligned with the via of theinsert.